Lecture - 2 - Thermal Evaporation
Lecture - 2 - Thermal Evaporation
Lecture - 2 - Thermal Evaporation
techniques
Thermal evaporation
Learning Outcomes
LO1: To have a scientific understanding of processing/deposition
techniques, such as: electrochemical, evaporation, sputtering,
chemical vapor deposition.
LO2: Ability to apply the knowledge and practical skills in the analysis
and solution of mechanical engineering problems.
LO3: An in-depth understanding of the ranges of properties and
characteristics exhibited by the materials.
LO4: To equip the students with skills necessary to carry out
research in advanced and emerging materials
LO5: Have an in-depth understanding of the principles and a
knowledge of the capabilities of the different types of analysis
covered in the module.
Thin film deposition methods
Thin film deposition - Vapor deposition methods
The deposition process of a thin film can be
divided into three basic steps:
Physical processes:
Evaporation/sublimation from a source Figure 3
Sputtering from a target CVD chemical vapour deposition
Figure 2 Sputtering
Figure 1- Thermal evaporation https://en.wikipedia.org/wiki/Sputter_deposition
Steps in film formation
Thin films go through several distinct stages during growth,
each affecting the quality of thin film and hence it’s physical
properties.
1. Adsorption
2. Nucleation
3. Island formation
4. Coalescence
5. Continuous film
http://www.tf.uni-kiel.de
Classification of SURFACE COATING
techniques:
Evaporation
Sputtering
Evaporation
Sputtering
http://physics.taskermilward.org.uk/KS4/core/html/evaporation.htm
Thermal evaporation
Definition and brief history of thermal evaporation
Evaporation steps:
1. Source material is evaporated Step 1
2. Transport of particles to substrate
3. Condensation of particles on substrate/ Vapours of the
material that reach the substrate surface are deposited
Evaporation – physical fundamentals
Evaporation mechanism
For a surface atom the transition into the gas phase is possible if:
kinetic energy of individual atoms on the surface >> the bond-
breaking energy Еλ,
Assumption: all surface atoms have the same binding energy and can
change into the gas phase with the same probability
As the temperature of the source material is increased, the
material typically goes through the solid, liquid, and gas
phases.
https://www.chem.purdue.edu/
gchelp/liquids/vpress.html
Ideal gas law can describe the behavior of the gases under
vacuum/low pressure
PV= NkT = RT
P – pressure; V - volume ; T – temperature
N- Avogadro’s number
K – Boltzman constant
R ideal gas constant https://slideplayer.com/slide/8080458/
Clausius – Clapeyron relationship describe the connection between
temperature and vapour pressure for
both solid-vapour and liquid vapour phases:
𝑑𝑃 ∆ 𝑄𝐷
= (1)
Where: 𝑑𝑇 𝑇 (𝑉 𝑔 −𝑉 𝑠)
P saturation vapour pressure
T temperature of the evaporation material - phase transition temperature
QD heat of vaporization
Vg and Vs the volumes of the gas and solid phase
Vg>>Vs ; volume of vapor typically in order of magnitude larger than a solid or liquid
phase. 𝑑𝑃
If the gas is assumed ideal: =𝑑𝑙𝑛𝑃 ( 𝑑𝑒𝑟𝑖𝑣𝑎𝑡𝑖𝑣𝑒𝑜𝑓 𝑙𝑛𝑃 )
𝑃
PV = RT where we consider =
𝑑𝑇
𝑇
2
=− 𝑑
1
𝑇 ( )
B integration constant
A constant depending on the heat of vaporization and on evaporation material
Vapour pressure of selected elements
too fast evaporation - the vapour pressure over the source is too large, vapour
particles collide with each other.
Large number of collisions at the substrate surface, a part returns to the
evaporation source.
too high temperature of evaporation source creates vapour bubbles. The
evaporation material is ejected by splashing out of the evaporation source,
which arrives partially on the substrate, which leads to film damage.
If the distribution of
the vapour stream is
constrained by cold
parts of the crucible
wall, then the
influence on the
vapour stream
density distribution is
called a chimney
effect.
Technical description of various types of
evaporation source
http://www.chegg.com/homework-help/consider-air-
Evaporation - Resistive heating composed-nitrogen-molecules-n2-concentration-n-chapter-
1-problem-11-solution-9780072957914-exc
https://www.tedpella.com/vacuum_html/vacuum-evaporation-
sources.htm
Evaporation - Electron beam evaporation
High melting point materials such as Ti (16680C) can be evaporated
locally with an Incident electron beam.
E-beam evap.
1. Electrons are emitted from a small and very hot filament
2. The electron beam is magnetically directed to the target/source material.
3. Electrons are smashed into a crucible containing the source material
4. Constant flow of electrons into material heats it until evaporation takes place
5. Electron bombardment heats very effectively allowing deposition of very high
temperature materials http://www.wikiwand.com/en/Electron_beam_physical_vapor_deposition
Resistive heating E-beam
Step2
http://edge.rit.edu/edge/P14651/public/Miscellaneous
2. Transport of particles to substrate
The evaporated atoms should have to compete with the residual gas atoms
Residual gas conditions:
1. The ration between residual gas and evaporated atoms should be small
2. The evaporated particles should move on straight lines
Mean free path – average distance between two collisions
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/menfre.html
http://www.iestechsales.com/blog-0/bid/64236/High-Vacuum-Conductance-Part-1-Bigger-is-Better
2. Transport of particles to substrate
Why Use Vacuum for Thin Films Deposition?
1.To increase the Mean Free Path
Vacuum in the chamber mean that free path of atoms in the chamber is
greater than the distance between the source and the substrate. High
vacuum is required to minimize collisions of source atoms with background
species.
1. Adsorption
2. Nucleation
3. Island formation
4. Coalescence
5.Thin film
l=h l = 2h
Micro and nano Fabrication, Tools and Processes,
d = thickness Hans H.Gatzen, Volker Saile, Jung Leuthold
d0 = maximum thickness at the substrate’s centre l=0
Less freedom of pattern/features current limitation !
Pattern Description:
Four, 1mm wide tracks with a length of 22mm. The
tracks are on a 4mm pitch. 1mm squares are also
included for alignment purposes
Coverage on steps and trenches current limitation !
Line of sight deposition means atoms travel in a straight line, generally bottom to
up vertical line, results in a poor step coverage
100 nm
thickness layer
https://www.olympus-lifescience.com/fr/microscope-
http://www.4college.co.uk/a/Cd/spect.php resource/primer/lightandcolor/mirrorsintro/
Thin film coatings
Application Material
Optical coatings Dielectric oxides SiO2; Ta2O5; Nb2O5
Metal carbides GeC
Micromechanical devices Ferroelectric oxides Fe
Metallization metals W,
Erosion protection Metals
Adhesion layer Metals Ti
Corrosion resistant coatings Metal carbides Ti, Ta
V carbide
Corrosion protection Metals Cr, Ti
Metal carbides Cr7C3; Cr3C2
Wear resistant coatings Metals/metal nitrides Mo, TiN
Metal carbides Cr7C3; Cr3C2
Friction reduction coatings metal nitrides TiN
Decorative coatings metal nitrides TiN
Hard coatings for machine tools / tool coatings metal nitrides ZrH, HfN
Metal carbides WC, W2C; W3C
Hard coatings for cuttings and milling Metal carbides TiC
Oxidation resistant coating for composites; Metal carbides HfC
coatings for superalloys
Protective coatings Metal carbides Ta and Nb carbide
2. Transport of particles to substrate
Mean free path – average distance between two collisions
Sputtering
Chemical Vapour Deposition
Books
for next lecture
“The Materials Science of Thin films” M.Ohring,